Traditional friction locks in knee prosthesis will lock when there is a load acting on them, i e when some part of the body weight rests on the artificial knee. This takes place in a still standing position as well as during the walking phase at heel strike when the heel hits the ground, and during toe off supporting oneself on the toe while extending the leg to initiate the swing phase when the leg swings freely in the air.
At normal walking without a prosthesis one starts already during toe off to flex the knee-joint to initiate the swing phase before all body weight is has been removed from the leg in question. This is not possible with knee prosthesis with a friction lock of the conventional kind. This involves an unnatural walking and makes walking in stairs and broken ground and cycling difficult since the knee prosthesis will lock as soon as it is loaded.
Knee prosthesis with friction locks in the form of a brake drum are disclosed in U.S. Pat. No. 4,206,519 and 4,351,070. In the last mentioned document there is a linkage transferring motions in the hip axis to the locking device in order to control the locking function thereof in response to the torque of the hip axis.
WO 97/10781 discloses a knee prosthesis with a friction lock in the form of a resiliently deformable substantially C-shaped member cooperating with an axle. The C-shaped member can be rotated about the axle in unloaded position, but in a locked position be deformed and locked to the axle.
The subject and most important features of the invention
It is an object of the invention to provide a knee prosthesis of the kind mentioned above, which permits rotation of the knee-joint in unloaded position and when it still is under body load during toe off when supporting oneself on the toe while extending the leg to initiate the swing phase, but which locks against rotation from extended to flexed position at other loaded positions, i e during heel strike when supporting oneself on the heel and during mid stance when supporting oneself on the whole foot. This has according to the invention been achieved by the fact that the second axle is located at a distance behind the first axle and is arranged to act upon the locking device in such a way that when the line of action from a load on the knee prosthesis passes through the second axle or between the first and the second axle, said second axle will act upon the locking device to activate it, while when said line of action passes through the first axle or in front of it the second axle will be unloaded and the locking device inactivated.
A further important advantage of the invention is that the knee-joint has a freewheel effect in such a way that it can always rotate from flexed to extended position also under load. By this for example walking in stairs will be possible. The knee-joint will however lock immediately again if it is rotated in the opposite direction, i e from extended to flexed position, under such load that the locking device is activated.